[Benio]

Here I present util how to handle with Pandemic mechanic, since I was unable to find any solution.
With a help of #WoWUIDev, this is the Pandemic util I made for my indev Warlock addon (note that the following util uses global Tooltip, but it's just for clarity):

Tooltip = Tooltip or {};
local Tooltip = Tooltip;
 
Tooltip._tooltip = 'Tooltip._tooltip';
Tooltip._tooltip_frame = CreateFrame('GameTooltip', Tooltip._tooltip, nil, 'GameTooltipTemplate');
 
-- @param {link} itemLink link to item
-- @return {object} text object of line: text from given @itemLink Tooltip
-- @credits Semlar#WoWUIDev
Tooltip.GetTooltip = Tooltip.GetTooltip or function(itemLink)
    Tooltip._tooltip_frame:SetOwner(UIParent, 'ANCHOR_NONE');
    Tooltip._tooltip_frame:SetHyperlink(itemLink);
    
    local O = {}; -- output
    for i = 1, Tooltip._tooltip_frame:NumLines() do
        local line = _G[Tooltip._tooltip ..'TextLeft' ..i];
        local text = line and line:GetText();
        if not text then break end;
        O[i] = tostring(text);
    end;
    
    return O;
end;
 
-- @param {uint|name} itemID item id or itemName item name
-- @return {number|nil} realDuration real spell duration (unaffected by pandemic) or nil if unknown
-- @credits Xuerian#WoWUIDev
Tooltip.GetSpellDuration = Tooltip.GetSpellDuration or function(itemNameORID)
    local link = GetSpellLink(itemNameORID);
    local text = Tooltip.GetTooltip(link);
    for i = 1, #text do
        local duration = string.match(text[i], "over (%d+%.?%d?) sec");
        if duration ~= nil then
            return tonumber(duration);
        end;
    end;
    
    return nil;
end;
 
-- @param {string} unitID unitID
-- @param {name} itemName item name
-- @param {string} rank UnitAura's rank
-- @param {string} filter UnitAura's filter
-- @return {bool|nil} pandemic if aura is affected by pandemic or nil if unknown
Tooltip.GetUnitAuraPandemic = Tooltip.GetUnitAuraPandemic or function(unitID, itemName, rank, filter)
    local realDuration = Tooltip.GetSpellDuration(itemName);
    local _, _, _, _, _, duration = UnitAura(unitID, itemName, rank, filter);
    
    if
            duration == nil
        or  realDuration == nil
    then return nil end;
    
     -- we love float, doesn't we?
    if math.abs(realDuration - duration) < 0.01 then return false end;
    if math.abs(realDuration *1.3 - duration) < 0.01 then return true end;
    
    return nil;
end;

The idea behind this method is to read the spell duration directly from tooltip, using Tooltip.GetSpellDuration, ex.:

local realDuration = Tooltip.GetSpellDuration("Siphon Life");

The cache it and whenever checking UnitAura for the same spell, we can use cached value if it's not nil, ex.:

local _, _, _, _, _, duration = UnitAura("target" "Siphon Life", nil, "PLAYER HARMFUL");
duration = realDuration or duration;

Finally, I've added a handy Tooltip.GetUnitAuraPandemic that takes same arguments like UnitAura, and returns if Spell duration already benefits from Pandemic or not.
Example: Before casting Siphon Life on target:

[15:20:07] Dump: value=Tooltip.GetUnitAuraPandemic("target", "Siphon Life", nil, "PLAYER HARMFUL");
[15:20:07] empty result

After casting first Siphon Life on target:

[15:20:12] Dump: value=Tooltip.GetUnitAuraPandemic("target", "Siphon Life", nil, "PLAYER HARMFUL");
[15:20:12] [1]=false

After casting second Siphon Life on target:

[15:20:15] Dump: value=Tooltip.GetUnitAuraPandemic("target", "Siphon Life", nil, "PLAYER HARMFUL");
[15:20:15] [1]=true